In joint prostheses or artificial joints it is common to insert one member of the artificial joint into a bone forming that joint. In the case of an artificial hip joint, that member can have a shaft which is forced into the marrow cavity of the femur and is provided at its upper end with a fitting adapted to receive a ball of the joint or forming the ball itself. Between the ball and the shaft, a closure plate can be provided which is adapted to close off the open section of the neck of the femur which may be cut away upon removal of the femur head to accommodate the artificial joint.
The shaft and the plate are generally composed of metal and it has been proposed to provide at least the shaft with a coating or layer of an artificial bone material, sometimes referred to as synthetic bone or as artificial bone replacement material.
The shaft and the closure plate can be formed together with the ball-forming or ball-connecting member in one piece from a titanium alloy or a cobalt-chromium alloy.
The joint prosthesis can be made by forging, casting and/or machining operations.
The layer of the so-called synthetic bone material is generally sintered on the metal body. The synthetic bone material can have a ceramic character and generally comprises at least one artificial apatite in the form of hydroxyapatite Ca.sub.5 OH(PO.sub.4).sub.2.
The material can be applied in a multilayer configuration and the various layers can be composed of different materials.
The layer of the synthetic bone material is not only highly compatible with natural bone tissue, but has a certain porosity.
The shaft which is inserted into the marrow cavity can be provided with ribs running in the direction of insertion so as to increase the surface area of the layer exposed to the bone tissue. Furthermore, the metallic surface of the shaft and the closure disc can be treated by spark-discharge erosion to form micropits or roughening formations which promote attachment of the synthetic bone material thereto.
A joint prosthesis as thus formed and constituted is not cemented into or onto the bone as is the case with some artificial joints, but rather, after insertion, becomes permanently attached by a growth of the natural bone tissue within the cortex of the bone into the layer.
The prosthetic joint of course must have sufficient alternating bending strength and should be sufficiently strong as to resist rupture under even the most extreme conditions of supporting the body.
In practice, the synthetic bone layer has been applied to the metal prosthesis body with a uniform thickness over the entire coated surface thereof. In practice, moreover, it has been found that the growth of natural bone material into the layer and hence the attachment of the prosthesis to the bone is unsatisfactory. Investigations have shown that this unexpected phenomenon appears to be a result of the fact that movements of the patient cause higher stresses on the bone and the connection between the bone and the prosthetic member in the region of the closure plate than elsewhere along the shaft remote from the closure plate. In other words the more highly stressed region of the prosthetic member and the bone and the more highly loaded tissue at the closure plate and directly adjoining same along the shaft does not permit as effective growth of the bone tissue into the layer and as effective bonding of the prosthesis in this region as in regions of reduced loading and less stress more remote from the plate.
The phenomenon, therefore, which has been recognized as a lack of effective bonding uniformly between the bone tissue and the implant, has been traced to the sharp differences in loading along the shank or shaft of the implant. This is especially the case with hip prostheses.